Variable inductance device



Aug. 15, 1961 cs. L. MILLER VARIABLE INDUCTANCE DEVICE 4 Sheets-Sheet 1 Filed March 26, 1956 F' I6. I a

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AT TORN EY 15, 1961 G. L. MILLER 2,996,682

VARIABLE INDUCTANCE DEVICE Filed March 26, 1956 4 Sheets-Sheet 2 500K SEMI F I 5 2| osc. 4 2 2 23 19 W6 17 INPUT TUN D DET OUTPUT 2o AMPL.

7 F I osc :DOUBLER 9 4 T TUNED PHASE o T INPU g CONSCIOUS OUT PU AMPL. DET. -o O 3 1 OUTPUT 13 7 24 25 F '6. 8 OSC. DOUBLER 1,2 193G PHASE 27 AMP CONSCIOUS DETECTOR 2O CURRENT SOURCE LOAD OR AM P 2 REGULATOR |NVENTOR SO I I 29 GABRIEL 1.0mm nnm ATTO RN EYS Aug. 15, 1961 G. L. MILLER 2,996,632

VARIABLE INDUCTANCE DEVICE Filed March 26, 1956 4 Sheets-Sheet 3 6 7 32 WE A4 F'IG.12. 42

INVEN TOR v GABRIEL LORIMER mum QS)FM)BMHWMWU AT T OR NEY 5 Aug. 15, 1961 G. L. MILLER VARIABLE INDUCTANCE DEVICE 4 Sheets-Sheet 4 Filed March 26, 1956 W H.T.

I NV E N T O R mam, 1.0mm MILLER Y MW", DMMQNHMJJM ATTORNE Y United States Patent 2,996,682 VARIABLE INDUCTANCE DEVICE Gabriel Lorimer Miller, London, England, assignor to National Research Development Corporation, London,

England Filed Mar. 26, 1956, Ser. No. 574,031 Claims priority, application Great Britain Apr. '1, 1955 9 Claims. (Cl. 3308) This invention relates to electromagnetic devices having a coil provided with a core of ferromagnetic material the reluctance of which core, in the direction of its longitudinal axis, is controllable.

The object of the present invention is to provide a new, useful and improved device of the type mentioned above together with new, useful and improved circuit arrangements in which the device is used.

According to the present invention, an electromagnetic device comprises a ferromagnetic core, an electric coil wound round the core with its axis in the direction of the longitudinal axis of the core and means for creating a magnetic field perpendicular to the longitudinal axis of the core.

The effect of the magnetic field perpendicular to the longitudinal axis of the core is to increase the reluctance of the core in the direction of the longitudinal axis, thereby reducing the inductance of a coil wound on the core or, if a plurality of coils is Wound on the core, to reduce the coupling between them.

The magnetic field perpendicular to the longitudinal axis of the core may be set up by a current caused by conduction or induction to flow in a conductor arranged inside the core along its longitudinal axis. In this case, the field is a circular field centered on the conductor. Alternatively, the field may extend transversely of the core.

In order that the invention may be clearly understood and readily carried into effect, a number of embodiments will be described in detail below, by way of example, with reference to the accompanying drawings, of which:

FIGURES 1a and 1b show a plan view and the cross section view respectively of an electromagnetic device of the type referred to in the opening paragraph, having three coils of part-toroidal form wound thereon;

FIGURE 2 shows diagrammatically, in cross section, a practical arrangement using a device as shown in FIG- URES 1a and 1b;

FIGURE 3 is a schematic representation of the arrangement shown in FIGURE 2 in the form in which the device is represented in the circuit diagrams shown in FIGURES 4 to 10;

FIGURE 4 shows a magnetic amplifier using the device in FIGURES 1a and 112;

FIGURE 5 shows in block schematic form, a magnetic modulator using the device;

FIGURE 6 shows a modification of the circuits of FIGURES 4 and 5 in which the output is related to the input both in amplitude and sign;

FIGURE 7 shows a modified circuit arrangement having a high-impedance input;

FIGURE 8 shows a circuit arrangement for using the device in a servo system;

FIGURE 9 shows a detail modification of the circuit arrangement shown in FIGURE 8;

FIGURE 10 shows a part of a circuit illustrating the use of the device as a switching arrangement;

FIGURE 11 shows a modified form of the device of FIGURES 1a and 1b, the central conductor being broken and brought out to two terminals;

FIGURE 12 shows diagrammatically a further modification of the device using circular mumetal laminations instead of ferrite material.

FIGURE 13 shows a form of the device in which the core of ferromagnetic material is saturated by a unidirectional field and FIGURE 14 shows diagrammatically an arrangement in which the core saturating field extends transversely of the core.

In FIGURES 1a and 1b, the magnetic device shown comprises a central conducting ring 1 of non-ferromagnetic material, in this case copper, having a mean radius of one centimetre and of one millimetre cross sectional diameter. Surrounding the conductor is a ferrite body 2 of wall thickness 0.5 mm. giving an overall cross sectional diameter of 2 mm. Wound on the annulus so formed are three equally spaced part-toroidal coils 3, 4 and 5, the input winding 3 comprising 1,000 turns of 45 SWG wire and the windings 4 and 5 each having turns of 36 SWG wire.

As an alternative, to give a very low impedance input, the winding 3 may consist of a single turn of heavy gauge copper wire. The windings 3, 4 and 5 need not occupy separated positions on the annulus, but may comprise three superimposed toroidal coils suitably insulated.

It is desirable that these coils should not appear as a single turn in the plane of the annulus and to this end, each coil has an even number of layers wound in alternate directions relatively to the longitudinal axis of the core.

The variation in reluctance of the ferrite annulus 2 linking with the three coils is achieved by saturating the magnetic material by means of a current flowing through the central conducting ring 1. In the device shown in FIGURE 2, the circulated current is induced into the central ring by a coil connected to an oscillator so that a field is set up in the ferrite annulus, the lines of force of which follow a circular path centred on the conducting ring viewing the device in cross section. When a sufficiently high current flows through the ring the ferrite material becomes saturated around this path and the reluctance in the direction of the longitudinal axis around the magnetic circuit, viewing the annulus 2 in plan, is correspondingly increased.

In the arrangement shown in FIGURE 2, the device shown in FIGURES la, 1b is enclosed in a high-frequency magnetic core 6, which in this particular case is a Mullard type LA3 pot core assembly of approximately 28 mm. overall diameter and 18 mm. height. As shown in the drawing, this assembly comprises a disc-shaped base with a central cylindrical rod projecting upwards from the centre, a hollow cylindrical body and a disc-shaped lid, the whole assembly being adapted to form a closed magnetic core with a central limb as shown in transverse section in FIGURE 2. In the annular cavity of this core there is arranged, together with the device of FIG- URES 1a, 1b, a centre-tapped coil 7 having 30 turns of copper wire, to which may be supplied a cyclically varying current.

This whole (device is represented schematically in FIGURE 3, corresponding elements in FIGURES 2 and 3 being represented by the same reference numerals.

'In the arrangement shown in FIGURE 4, the device of FIGURE 2 is used as a component in a magnetic amplifier. The coil 7 is connected at one end to the anode of an oscillator valve 8, and at the other end, through a capacitor 11, to the grid of the valve. The coil 7 is tuned by a parallel capacitor 9 and the centre tap of the coil 7 is connected to the I-I.T. positive supply. A grid leak 10 is connected from the grid of the valve 8 to its cathode.

The output winding 4 of the device is tuned by a parallel variable capacitor 12, and one end is connected to an output terminal 18. The other end of the coil 4 is connected through a rectifier 13 to a low-pass filter comprising a series resistance 14 and shunt capacitors 15 and 16. The resistance 14 is connected to the other output terminal 17.

The tuned circuit 7, 9 is adjusted to resonate at near the highest frequency at which the core assembly can operate and, in the example given, this may conveniently be 500 kc./s.

A circulating current is induced in the ring 1 by the coil 7, of such magnitude that the ferrite annulus 2 is saturated at least by the peak current in each half cycle of the oscillator current. That is to say, the core is saturated at a frequency of l mc./ s.

If a direct current is applied to the input winding 3 at the terminals 19, 20, the flux set up in the annulus 2 is reduced, as the reluctance of the annulus is increased, at the frequency of 1 mc./s. Therefore, a distorted alternating current of frequency 1 mc./s. is induced in the coil 4 which is tuned to resonate at this frequency by the capacitor 12. This alternating voltage is rectified by the rectifier 13 and the filter 15, 14, 16' serves to remove the ripple components. The direct current output appearing at the terminals 17, 18, then bears a linear relationship to the direct current applied at the terminals 19, 20, within the limits corresponding to the saturation in either sense of the ferrite annulus along the path coinciding with the conducting ring 1.

As a modification of the arrangement shown in FIG- URE 4, the single rectifying element 13 may be replaced by a bridge-connected rectifier using crystal diodes.

FIGURE shows a modified arrangement of the circuit shown in FIGURE 4 arrange to operate as a magnetic modulator. Coil 7 is connected to a 500 kc./s. oscillator shown at 21. The coil 4 is connected to the input of a tuned amplifier, tuned to 1 mc./ s. and shown at 22, the rectified output of which appears at the output terminals 17 and 18.

In this circuit arrangement, the device operates as a high-gain magnetic modulator, the output current at the terminals 17 18 bearing a linear relationship to the current applied at the input terminals 19 and 20, within a frequency range from DC. to at least 200 lcc./s., and the arrangement has the advantage that it does not show zero drift.

In the circuit arrangements shown in FIGURES 4 and 5, it is immaterial in which direction the current flows in the input coil 3. In order to obtain information in the output regarding the sign of the input current, it is necessary to use a modified circuit arranged as shown in FIG- URE 6.

In FIGURE 6, an oscillator 24 operating at 500 kc./s. is connected to the coil 7 and also to the input of a frequency doubler 25. The output coil 4 of the device is connected to a tuned amplifier 26 tuned to 1 mc./s. as in the previous arrangement, and the output of this tuned amplifier is connected to one input of a phase-conscious detector 27, the other input of which is fed from the output of the'frequency doubler 25. In this arrangement, the output appearing at the output terminals 17, 18 corresponds both in magnitude and sign to the input current applied at the terminals 19, 20.

It will be appreciated that the input to the coil 3 in the previously described circuit arrangements is a relatively low impedance input. If a high impedance input is required, the arrangement shown in FIGURE 6 may be modified by connecting the magnetic device as shown in FIGURE 7.

In FIGURE 7, the input terminals 19, 28- are connected respectively to the grids of two triodes 30, 31, the anodes of which are connected together and to: a HT. positive supply. The ends of the coil 3 are connected through resistances 28 and 29 to the input terminals 19, 20 respectively, and through resistances 32 and 33 respectively to the cathodes of valves 30, 31. Two resistances 34 and 35 are respectively connected from resistances 32 and 33 to the negative pole of the HT. supply. The magtion shown in FIGURE 9.

netic device is otherwise connected as shown in FIGURE 6, the terminals shown at 17, 18 being connected to the input of the tuned amplifier 26.

The device may conveniently be used in a servo-control arrangement, for which purpose three coils are wound on the annulus 1, '2, two: of the coils being wound with an appropriate number of terms so that the magnitude of two currents can be compared by being passed in opposite senses through the two coils and an error or difference signal is then derived from the third coil. That is to say, the currents to be compared need not be of equal magnitude, but, in such a case, the corresponding coils are wound with the appropriate number of turns to produce equal fluxes around the annulus 1, 2. If alternat: ing currents are to be compared, they must be in phase.

Such an arrangement is shown diagrammatically in FIGURE 8 which, it will be seen, is a modification of the circuit arrangement shown in FIGURE 6, in which the output of the phase-conscious detector is connected to the input of an amplifier 28 the output of which is used to control a current flowing from a current source or regulator 29 through a load 30. Included in the load circuit is the coil 5, so that, by this arrangement, the load current flowing through the coil 5 is compared with a reference current applied at the terminals 19, 20 and flowing through a coil 3.

The advantage of this servo-control system lies in its speed of response and in its high sensitivity. For example, a significant error signal from the coil 4 is derived from a current difference of 1,uA, between the currents flowing in the coils 3 and 5, when these coils are provided with 100 turns each.

As an alternative to using a DC. reference current flowing through the coil 3, a flux in the annulus 1, 2 produced by a permanent magnet may be used to the same effect, if so desired.

Such an arrangement is shown in the detail modifica- In this figure, the annulus 1, 2 passes through gaps or apertures in the parallel arms of a horseshoe magnet 31. A steady magnetic flux is set up the direction around the annulus 1, 2, in the same manner as by the coil 3 in the arrangement according to FIGURE 8; a magnetic shield 32 is employed, as indicated in the figure, to screen the field of the permanent magnet 31 from the remainder of the magnetic circuit.

The advantage of this arrangement over that shown in FIGURE 8 lies in the constancy and the mobility of the reference, namely, the permanent magnet 31.

A system substantially as shown in FIGURE 9 has been used to control a 30 kw. Metadyne, in which system it was found that the overall stability was of the order of 1 in 10 with a response time to error signals of one millisecond.

The device may further be used as a switching transformer and such an application will now be described with reference to FIGURE 3. In this arrangement, the input is connected to the coil 3 and the output to the coil 4, so that the path between input and output lies through the magnetic circuit provided by the annulus 1, 2. By applying a switching pulse across the coil 7, a current is induced in the conducting ring to saturate the ferrite annulus 2. so as effectively to destroy the coupling between the coils 3 and 4. The switch thus functions as a normally closed switch which is open circuited by the application of a switching pulse.

This switching arrangement offers favourable characteristics in that it is quick-operating, giving a switching time of 1 millisecond or better, and in that the switching pulse itself does not appear at the output terminals of the device.

The arrangement, or a number of such arrangements, may, for example, be used for head switching of the read-write heads of a magnetic memory device in an '5 a muting pulse to switch off those heads which are not required to operate.

The device may further be adapted to provide a switching arrangement where the switch is normally open and is closed by the application of a switching pulse. Such an arrangement is shown diagrammatically in FIGURE 10, in which the arrangement shown in FIGURES 2 and and 3 is connected in a bridge circuit balanced by a corresponding inductance 33 and having the remaining arms comprising resistances 34 and 35. The input terminals of the device are connected respectively to the junctions of the inductances and the resistances, and the output terminals 38, 39 are connected across the other diagonal of the bridge. In this arrangement, a pulse or other signal applied on the input terminals 36 and 37 is balanced by the values of the bridge so that no signal appears across the terminals 38, 39. By the application of a switching pulse across the coil 7, the reluctance of the device is altered and the bridge unbalanced, so that an output signal then appears across the output terminals 38, 39.

Such an arrangement may be used for a matrix parallel-access switching system for electronic computer memory drums.

Although the magnetic device used in the foregoing circuit arrangements is conveniently of the general form described with reference to FIGURES 1a, 1b and 2, various modifications of the device are permissible While retaining at least in part, the features of the device and circuit arrangements described. For example, as an alternative to having a closed conducting ring embedded in the ferrite annulus 2, the conducting ring may be broken and brought out to terminals 40, 41 as shown in FIGURE 11. If such an arrangement is used, current is supplied to the ring 1 from a current source instead of being induced therein, as in the arrangement described with reference to FIGURES 1a, 1b and 2.

Alternatively, the single turn conductor shown in FIG- URE 11 may be replaced by a number of turns, insulated from one another and embedded in the ferrite annulus near its centre. With a device of this form, the saturation of the ferrite material may be effected by a smaller current supplied, for example, from a high impedance device or from a D.C. source.

As a further alternative, a magnetic material such as mumetal may be used instead of a ferrite material and in this modification, one form of which is shown by way of example in FIGURE 12, the core may be a series of superimposed circular washers 42 of mumetal separated by suitable insulating layers 43.

Such a device would not be suitable for a high frequency of 500 kc./s. as described for the ferrite device, and an oscillator of say 20 kc./s. would be connected instead across the coil 7. This limitation would result in a loss of sensitivity in the arrangements described with reference to FIGURES 4 to 10.

As a further alternative construction of the device, mumetal wire may be used, the strands being separated by insulating material and bundled together to form an annulus.

Circuit arrangements in which an output is obtained which corresponds to the input current in both amplitude and sense are shown in FIGS. 6 and 8. Both these circuit arrangements use a phase-conscious detector to provide sensing. An alternative method of providing sensing without the use of a phase-conscious detector consists in providing a unidirectional saturating field for the ferrite annulus.

One such circuit arrangement is shown in FIGURE 13, in which the D.C. input terminal 19 is connected to the coil 3 through a RF, choke 44, the other input terminal 20 being connceted directly to the coil 3. Two A.C. output terminals 17, 18 are connected directly to the ends of the coil 3. The coil 7 of the device is supplied with unidirectional current pulses from a thyratron circuit.

To this end, a thyratron 45 has its anode connected to a positive voltage supply through a resistor 46 and has its cathode connected to the negative terminal of the supply [through resistors 47 and 48 connected in series. The anode is connected to the same negative supply terminal through a resistor 49 and a capacitor 50 connected in series. A capacitor 52 is connected between the thyratrcn cathode and the junction of resistors 47, 48 and is fed from the positive voltage supply, through a resistor 51. The coil 7 is connected between the junction of resistors 47, 4S and the negative terminal of the voltage supply.

With a direct current flowing between the input terminals 19, 20, an A.C. output current is obtained in the form of pulses of the same general shape as the pulses supplied to the coil 7. If the sense of the input current is reversed, the sense of the output pulses is also reversed, other conditions remaining constant. In either case the output pulse amplitude is proportional to the magnitude of the input current.

In the devices described with reference to FIGURES 1 to 11, the saturating field current is induced or introduced into the control conductor 1 within the ferrite annulus. In such an arrangement, the saturating field is a circular field centred on the conductor. In the device shown in FIGURE 12 and in other devices to be described below, the central conductor is dispensed with and a transverse saturation field in the direction of the axis of the annulus is used.

One such arrangement is shown diagrammatically in FIGURE 14 in which a Mullar-d type FX1508 ferrite annulus 55 has a toroidal coil 56 of 25 turns of 47 SWG wire wound thereon. The core 55 and coil 56 is arranged in the gap of a Mullard type LA. 6 pot core 53, or coil 54, being arranged about the central member.

Unidirectional pulses are supplied to the coil 54 of sufiicient magnitude to create an axial field in the gap of the pot core 53 to saturate the ferrite annulus '55. The arrangement then operates in an analogous manner to that described with reference to FIGURE 13, the coil 54 replacing the coil 7, the D.C. input current being supplied to the coil 56, in place of the coil 3, and the A.C. output being derived from the coil 56.

The coil 54 may alternatively be supplied with a sinuoidal current, in which case the A.C. output current corresponds to the D.C. input current in amplitude, and its phase relationship to the saturating field current is indicative of the sense of the D.C. input current. This output current may, if desired, be supplied to a phase conscious detector in the manner described with reference to FIGURES 6 and 8.

What is claimed:

1. A magnetic amplifier comprising a first closed mag netic core, a second closed core of ferromagnetic material arranged around a part of said first closed magnetic netic core, a closed conducting loop arranged inside said second core, a first and a second coil wound on said second core with their axes in the direction of the longitudinal axis of said second core, an induction coil wound on said first core, an oscillator connected to supply a sinusoidally varying current to said induction coil, a voltage source supplying a voltage to be amplified connected across said first coil, a capacitor connected across said second coil to tune said second coil to double the frequency of said oscillator, a rectifier connected to derive its input from said second coil and output terminals supplied from the output of said rectifier.

2. A magnetic amplifier comprising a first closed magnetic core, a second closed core of ferro-magnetic material arranged around a part of said first magnetic core, a closed conducting loop arranged inside said second core, -a first and a second coil wound on said second core with their axes in the direction of the longitudinal axis of said second core, an induction coil wound on said first core, a pulse generator connected to supply a pulsatory current to said induction coil, a voltage source supplying a 'voltage to be amplified connected across said first coil, a capacitor connected across said second coil to tune said second coil to the pulse repetition frequency of said pulse generator, a rectifier connected to derive its input from said second coil and output terminals supplied from the output of said rectifier.

3. A magnetic amplifier comprising a first closed magnetic core, a second closed core of ferromagnetic material arranged around a part of said first closed magnetic core, a closed conducting loop arranged inside said sec ond core, a first and a second coil Wound on said second core With their axes in the direction of the longitudinal axis of said second core, an induction coil wound on said first core, an oscillator connected to supply a sinusoidally varying current to said induction coil, a voltage source supplying a voltage to be amplified connected across said first coil, a capacitor connected across said second coil to tune said second coil to double the frequency of said oscillator, a phase-conscious rectifier having its first input connected across said second coil and said capacitor, a frequency doubler having input terminals connected to said oscillator and output terminals connected to the secnd input of said phase-conscious rectifier and output terminals supplied from the output of said phase-conscious rectifier.

4. In combination, a first magnetic core, a first coil wound round said first magnetic core, current supply means connected to supply a cyclically varying current to said first coil, a second magnetic core defining a closed magnetic circuit arranged around said first magnetic core, a closed conducting loop arranged inside said second core and passing round said first core, a second coil wound on said second core with its axis extending longitudinally of said magnetic circuit, means for establishing a magnetic flux longitudinally of said magnetic circuit, a pair or output terminals and an alternating current detector connected intermediately of said second coil and said output terminals.

5. The combination as claimed in claim 4, in which the said means for establishing a magnetic flux longitudinally of said magnetic circuit comprise third and fourth coils Wound on said second core with their axes extending longitudinally of said magnetic circuit.

6. The combination as claimed in claim 4, in which the said means :for establishing a magnetic flux longitudinally of said magnetic circuit comprise a third coil wound on said second core with its axis extending longitudinally of said magnetic circuit. 7

7. In combination, a closed core of ferrite magnetic material, a closed conducting loop embedded in said core, a first coil wound on said core with its axis extending in the direction of the longitudinal axis of said core and means for inducing a high-frequency circulating current in said closed conducting loop comprising a current source and a second coil connected thereto having its axis passing through said closed conducting loop, said first coil being connected in an alternating current circuit.

8. In combination, a high frequency magnetic core of the pot type having a central limb providing a closed toroidal magnetic circuit, a closed core of ferrite magnetic material arranged around the central limb of said pot type core, a closed conducting loop embedded in said ferrite core and passing around the central limb of said pot type core and means for inducing a high-frequency circulating current in said closed conducting loop comprising a current source and a coil connected thereto and arranged around the central limb of said pot type core.

9. The combination as claimed in claim 4, in which the said means for establishing a magnetic flux longitudinally of said magnetic circuit includes a permanent magnet having opposite poles spaced longitudinally of said magnetic circuit.

References Cited in the file of this patent UNITED STATES PATENTS 

